The present invention relates to an electrical stepping motor apparatus comprising an electrical detent means by which the motor shaft and associated load may be manually rotated to and held at any step position.
Stepping motor systems comprising multi-phase motors are known in the art. A typical application of such a system is for drivably rotating a platen in a printer, facsimile receiver or the like. A photoelectric transducer is connected to the motor shaft and generates a timing pulse each time the shaft is rotated by one step. For electrically rotating the shaft from a present step to a new selected step, the phases of the motor are individually energized in such a manner as to cause the shaft to rotate toward the new step. The pulses are counted and the shaft stopped when the new step is reached. One or more of the phases of the motor are energized in such a manner as to electrically hold the shaft at the new step position.
Although such a stepping motor drive system is generally advantageous and acceptable, a drawback has remained heretofore in that satisfactory means for manually rotating the shaft have not been proposed. Although it is possible to rotate the shaft against the energization of the motor phases to a new equilibrium position, this is not satisfactory since there are only a few equilibrium positions for each combination of energized phases. The angle between the equilibrium positions is equal to the number of motor phases times the angle between step positions. For a three phase motor in which the angle between adjacent steps is 15.degree., an equilibrium position exists for each 45.degree. of shaft rotation. In other words, although there are 24 steps per revolution of the shaft, there are only 4 equilibrium positions per revolution for each combination of energized motor phases. For this reason, although the shaft may be electrically rotated to and positioned at any step, the shaft may only be manually rotated to and positioned at intervals of three steps. This makes manual rotation of a platen connected to the shaft coarse due to the small number of electrical equilibrium or detent positions.
A prior art expedient to overcome this problem is to provide a clutch between the shaft and the platen which may be disengaged to rotate only the platen. Another expedient is to de-energize the motor when the selected step is reached and engage a mechanical detent mechanism. Yet another expedient is to provide a mechanical vernier mechanism for rotating the platen with the motor temporarily de-energized. Although such prior proposals constitute partially acceptable solutions, each involve the provision of additional mechanical components which add to the cost and complexity of the apparatus and do not constitute a detent means which allows the shaft to be manually rotated precisely to each step position.